Multi-leaf collimators (MLC) are commonly used in radiotherapy machines. Typically, an MLC includes two sets of independently adjustable leaves. Each leaf is thick enough to attenuate or block completely a beam of radiation. The first set of leaves is positioned on one side of the beam's path, and the second set mirrors the first set on the other side of the beam's path. The leaves can be positioned independently of one another to form an aperture in a shape like that of the area to be irradiated, so that only the targeted area is irradiated while surrounding areas are shielded.
Because adjacent leaves need to be able to move relative to one another, there is necessarily a gap between them. The leaves are designed so that one or more portions of one leaf overlap one or more corresponding portions of an adjacent leaf. Thus, the gap between adjacent leaves does not provide a straight-line passageway for the radiation beam. Instead, radiation that enters a gap will encounter a thickness of leaf material that is sufficient for attenuating or blocking the radiation. In this manner, leakage of radiation through the gaps to areas other than the targeted area is prevented.